1. Field of the Invention
This invention relates to a novel electrical insulating oil composition. More particularly, the invention relates to an electrical insulating oil composition which comprises compounds having biphenyl skeletons and is characterized in that the composition is excellent in low temperature characteristics and suitable for use in impregnating capacitors.
2. Description of the Prior Art
The compounds having biphenyl skeletons such as isopropylbiphenyl have been widely used as electrical insulating oils, especially those for capacitors.
There are three position isomers of o-isomer, m-isomer, and p-isomer in isopropylbiphenyls. The p-isomer is liable to separate out as crystals, so that it is proposed in U.S. Pat. No. 4,266,264 to increase the content of m-isomer. According to this proposal, the separating out of p-isomer is suppressed and, in this sense, the low temperature characteristic can be surely improved.
The isopropylbiphenyls are widely used as described above, however, the viscosity of them at low temperatures is high and the hydrogen gas absorbing capacity is not always good.
The evaluation of capacitors and the electrical insulating oils for impregnating them is done by testing partial discharge phenomenon. It is considered that the mechanism of the occurrence of this partial discharge correlates to hydrogen gas absorbing capacity. That is, hydrogen gas is generated prior to the occurrence of partial discharge. The hydrogen gas generated in the relevant portion is locally saturated in due course to produce bubbles of hydrogen gas and then the partial discharge occurs. In this procedure, when the quantity of hydrogen gas generation exceeds the capacity of the absorption or diffusion of hydrogen gas, bubbles are produced and finally the partial discharge is caused to occur as described above. Accordingly, in order to increase the quantity of absorption or diffusion, it is required of electrical insulating oils that hydrogen gas absorbing capacity is large and the diffusion of hydrogen gas is good enough.
The hydrogen gas absorbing property itself relates to the molecular structures of insulating oils. The absorption and diffusion of hydrogen gas relates to the viscosity of liquid rather than molecular structures. That is, if an electrical insulating oil is of high viscosity, mass transfer is inhibited and, as a result, the absorption and diffusion of hydrogen gas are suppressed.
The isopropylbiphenyls are characterized in that the viscosity seriously increases a low temperatures. For example, it is said that the isomer mixture of isopropylbiphenyls containing much m-isomer difficultly causes the separating out of crystals, however, its viscosities are 4.6 cSt at 40.degree. C., 1500 cSt at -40.degree. C. and as high as 12,000 cSt at -50.degree. C. The absorption and diffusion of locally generated hydrogen gas are retarded seriously under such high viscosity conditions, therefore, the hydrogen gas is locally saturated to produce bubbles and finally vital dielectric breakdown is brought about.
Therefore, in view of only the viscosity, it cannot always be said that the isopropylbiphenyls are good in low temperature characteristic.
Accordingly, it is eagerly desired to propose an electrical insulating oil which has low viscosity at low temperatures and is excellent in hydrogen gas absorbing capacity.